1. Field of the Invention
The present invention relates to an electrophotographic apparatus, more specifically to an electrophotographic apparatus having a charging means provided in contact with an electrophotographic photosensitive member.
2. Related Background Art
In electrophotography, an image is obtained by use of an electrophotographic photosensitive member (hereinafter simply referred to as a "photosensitive member") containing a photosensitive material such as selenium, cadmium sulfide, zinc oxide, amorphous silicon, and organic photoconductive substances through basic processes of electric charging, light image exposure, latent image development, image transfer, image fixation, cleaning of the electrophotographic member, and so forth. The electric charging process is conventionally conducted by corona discharge caused by application of high DC voltage (5 to 8 KV) to a metal wire. This method, however, has disadvantages that the corona discharge products such as ozone and NO.sub.x deteriorate the surface of the photosensitive member to cause fogging and low quality of images; soiling of the wire impairs image quality to cause white blank or black stripes in the formed image; and so forth. In particular, a photosensitive member having a photosensitive layer composed mainly of an organic photoconductive material is less stable chemically, and is liable to deteriorate by chemical reaction (mainly by oxidation) on exposure to the corona products in comparison with the photosensitive members such as selenium photosensitive member and amorphous silicon photosensitive member. Therefore, this type of photosensitive member tends to deteriorate to cause fogging of images and decrease of copied image density during repeated use under corona charging, resulting in short printing life of the photosensitive member.
The corona charging is less efficient as the charging means because the electric current directed to the photosensitive member is only 5 to 30% of the entire current, and most of the current flows to the shield plate.
In order to offset such disadvantages, methods are investigated in which the surface of the photosensitive member is charged by application of voltage to the surface by use of a charging member provided in contact with the surface of the photosensitive member without employing a corona discharger, as disclosed in JP-A-57-178267, JP-A-56-104351, JP-A-58-40566, JP-A-58-139156, JP-A-58-150975, etc. ("JP-A" herein means Japanese Patent Laid-Open Application).
More specifically, the surface of the photosensitive member is charged to a prescribed potential by bringing a charging member like an electroconductive elastic roller into contact with the surface of the photosensitive member with application of DC voltage of about 1 to 2 KV to the charging member.
In spite of many proposals on such types of charging methods, few of them are commercialized because of non-uniformity of the charging, liability of dielectric breakdown of the photosensitive member, and other reasons. The non-uniformity of the charging signifies spot-like irregularity in the charging of the surface of the photosensitive member, which causes image defects such as white dots in normal development (white dots in a solid black image) or fogging in reversal development.
In order to improve the uniformity of the charging without the above problems, a method is proposed in which pulse voltage formed by superposition of AC voltage (VAC) to the DC voltage (VDC) is applied to the charging member to charge uniformly the surface of the photosensitive member (JP-A-63-149668).
In this method, in order to prevent image defects like fogging, white dots in normal development, or black dots in reversal development, the potential difference (V.sub.p-p) between the peaks of the AC voltage needs to be twice or more the DC voltage.
This method involves problems as follows. As the superposing AC voltage is raised to prevent the image defects, the maximum application voltage in the pulse voltage rises also, which tends to cause dielectric breakdown at slightly defective site in the photosensitive member. In particular, this tendency is remarkable in a photosensitive member employing an organic photoconductive material which has a relatively low dielectric strength. The dielectric breakdown of the photosensitive member will cause white blank (white stripes) in normal development and black stripes in reversal development in the length direction of the contact portion. A pinhole in the photosensitive layer give rise to leakage of the current through the pinhole portion to lower the applied voltage to cause white stripes or black stripes.
In a direct charging method, the application of pulse voltage causes noise generation owing to vibration between the charging member and the photosensitive member corresponding to the frequency of the applied voltage. This noise tends to become larger with the increase of the V.sub.p-p and of the frequency of the applied AC. This is also a serious problem.
JP-A-61-57958 discloses a uniform charging method. In this method, a photosensitive member employed has a protection layer having a resistivity controlled by electroconductive particles dispersed therein, the electroconductive fine particles are brought into contact with the photosensitive member, and voltage is applied directly to the electroconductive fine particles to inject electric charge into the protection layer, thereby uniform charging being attained.
In this method, the material constituting the electroconductive particles which are dispersed in the protection layer includes metals such as copper, aluminum, and nickel, and metal oxides such as zinc oxide, tin oxide, antimony oxide, and titanium oxide. The dispersion state of the particles affects greatly the uniformity of the electric charge injection. If the dispersion is poor, the charge injection becomes non-uniform to lower the chargeableness or to cause irregular charging. Therefore, uniform dispersion of the electroconductive particles is especially important in this method.
However, metal particles or metal oxide particles tend to agglomerate in a resin or in a resin solution. The formed dispersion of such particles, after it has been formed, is liable to cause secondary agglomeration or precipitation. Therefore it is extremely difficult to disperse well such electroconductive particles. In particular, fine particles (primary particle diameter of not larger than 0.3 .mu.m) or ultrafine particles (primary particle diameter of not larger than 0.1 .mu.m) of such a material, which are desired to be uniformly dispersed for achieving uniform chargeableness, exhibit more pronounced tendency of less dispersibility and less dispersion stability. Thus the fine particles has problems of low chargeableness and charge irregularity caused by less charge injectableness resulting from less dispersibility of the electroconductive particles, which causes deterioration of image such as irregularity and decrease of image density, and fogging of images.
In recent years, with the progress in image quality for sharpness of a latent image and use of finer toner particles, electrophotographic apparatuses are being investigated which are capable of giving more uniform charging.